1. Field of the Invention
The invention relates generally to welding. More specifically, the invention is directed to a method of welding metal pieces together using buttering and heat treating techniques to avoid post-weld heat treatment and to reduce or eliminate the heat-affected zone.
2. Description of Related Art
There are two aspects of welding that increase costs of welding and that lead to failure of welded components: the presence of a heat-affected zone (HAZ) and post-weld heat treatment (PWHT), which is used to address the problems presented by the HAZ. As is known in the art, the heat from a weld creates a HAZ in the metal adjacent to the weld. The creation of this HAZ has adverse metallurgical effects, such as the creation of notch effects or grain growth, which cause a weakening of the metal in the HAZ. While new alloys, for example, 2-12% chromium content by weight, have been developed to provide higher strengths for high temperature pressure applications compared to alloys and steels previously used, failures that limit the useful life of components produced from these materials tend to occur next to welds in the HAZ. In addition, attempts to develop filler materials to strengthen welds and reduce the effects of the HAZ have not been satisfactory.
Another method used to improve the metallurgical properties in the HAZ is a PWHT. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code requires that welding applications of low alloy steel piping and pressure vessels be subjected to a PWHT to achieve toughness, tensile and hardness properties in the weld HAZ. However, PWHT tends to be an expensive process requiring considerable time to perform. PWHT typically requires heating the welded metal piece to a temperature that is just below the first transformation temperature for the material. ASME-mandated PWHT of a high temperature material containing 2% chromium requires the material to be held at 1350° F. for one hour for each inch of material thickness. The ramp rate at which the temperature of the material can be raised to the holding temperature and cooled back to room temperature must be closely controlled and requires several hours to complete. Typical PWHT operations on a 2″ thick pipe manufactured from this material, including set up, time to raise the material to temperature, hold time, and cooling time can require 24 hours. Components with multiple welds may require several PWHT operations.
Fabricators of components generally perform such PWHT in a large oven that allows for several weld joints to be tempered concurrently. The physical size of the oven obviously limits the size of the components that can be tempered. Therefore, some PWHT procedures must be performed in the field or at the job site. In these cases, parts are welded together and then transferred to the field where a PWHT is performed. PWHT in such larger components are generally performed utilizing resistance pads or induction heat treatment equipment. As such, the number of weld joints that can receive PWHT at one time is limited by the availability of power and PWHT equipment. On large construction jobs, scheduling of PWHT also becomes an important task.
Moreover, simply moving components in the fabrication shop to the oven, storage, other fabrications areas or to the job site can result in failures in welds and HAZ areas. Assembly of components at the job site presents additional challenges to achieve acceptable PWHT. Air flow currents around the components, including, for example, both external and internal currents from wind and chimney effects, can result in the material not reaching sufficient temperature during PWHT to develop the required properties. Special care to support the components is also required during PWHT operations since the strength of the material is greatly reduced by the high temperature of the PWHT operation.
Even with PWHT, failures still occur near the weld or in the HAZ. Therefore, a need exists for an improved method of welding that reduces the effects of the HAZ and eliminates the need for PWHT.